The use of reciprocating internal combustion engines (ICE) dominates the sector of the in-the-road transportation sector, both for light and heavy duties. CO2 reduction is the technological driver, considering the severe worldwide greenhouse commitments. In ICE more than one third of the fuel energy used is rejected to the environment as thermal waste through the exhaust gases. Therefore, a greater fuel economy could be achieved, recovering this energy and converting it into useful electric power on board. Financial benefits will be produced in terms of fuel cost which will rebound similar benefits in terms of CO2 emitted. For long hauling vehicles, which run for thousands of miles, frequently at fixed engine operating conditions, this recovery appears very worthy of attention. In this activity, an ORC-based power unit was designed, built and tested fed by a heavy duty diesel engine, so contributing to the huge efforts on going in that specific sector. As already experienced by the authors in similar passenger car and light duty applications, important differences appear between theoretical predictions (copious in literature) and measured performances in real cases. Main testing attention, in particular, has been reversed to the expander, here represented by a single stage impulse axial turbine. An overall net efficiency of the power unit was around 4 %, with a mechanical power equal to 2 kW.